Why Are Oxygen Sensors Required?

Everyone agrees that the amount of pollution produced by motor vehicles should be reduced. In order to encourage this, governments have introduced tougher and tougher exhaust gas emission legislation.

The most popular method used by vehicle manufacturers is the three-way catalyst. This device converts the main pollutants in the exhaust gas to less harmful gasses. However, the three-way catalyst only works efficiently if the air-fuel ratio can be kept within very tight limits. This is where the oxygen sensor fits into the picture.

The oxygen sensor is typically situated in the exhaust pipe just before the three-way catalyst. The central element of the oxygen sensor is exposed to the exhaust gas.

The oxygen sensor continually detects the oxygen content in the exhaust gas. Its output signal is connected to the ECU and changes to indicate a rich or lean fuel mixture. Through its output signal, the sensor "informs" the ECU if the vehicle is running rich or lean. The ECU uses the information supplied to it by the sensor to decide whether it needs to strengthen or weaken the fuel mixture to achieve optimum air-fuel ratio. This is referred to as closed loop control because the output (oxygen content in the exhaust gas) is measured and fed back to the controller (ECU) which can then correctly control the fuel mixing system. By ensuring that the mixture strength is always correct both combustion and catalyst efficiency are optimized.

Operation of an Oxygen Sensor

NTK manufactures two general types of oxygen sensors. The first and more common type uses a zirconia ceramic sensing element and the second uses a titania element. The working principle of the two types of sensors is described in this section.

Zirconia Sensors

An important property of the Zirconia element is that it can conduct oxygen ions above a temperature of about 350 deg.C. When the sensor is fitted, the outside of the Zirconia element is exposed to the exhaust gas and the inside is in contact with air. Both sides of the element are coated with a thin layer of platinum that act as electrodes and carry the sensors' signal (voltage) from the zirconia element to the lead wire. At operating temperature, oxygen ions are able to pass through the element and deposit a charge on the platinum electrode thus generating the voltage signal.

A high signal voltage is generated across the electrodes when different levels of oxygen are present on the two sides of the element. Due to the properties of the Zirconia element there is a large change in sensor voltage when the air-fuel ratio (AFR) is 14.7.

If the AFR is low (rich fuel mixture) the sensor output voltage will be high because the electrode exposed to the exhaust gas is in contact which much lower amounts of oxygen than the one exposed to air. Conversely, if the AFR is greater than 14.7 (lean fuel mixture), the signal voltage will be low.

The ECU uses the voltage produced by the sensor to instruct the fuel mixing system to strengthen or weaken the mixture. A low voltage informs ECU that the fuel mixture is lean and a high voltage that the mixture is rich. The ECU can then take appropriate action to achieve optimum AFR.

The sensor only produces a voltage when the element is above approximately 350 deg.C and it takes the exhaust gas a little while to heat the element up to this temperature after the engine has been switched on. In order to reduce the time it takes for the sensor to reach working temperature, most sensors today are fitted with an internal ceramic heater. These sensors have 3 or 4 lead wires. In the 3 - wire type, two of the wires are required to supply the heater, one wire is used to carry the sensor signal and signal ground is achieved through the manifold. The fourth wire in 4 - wire types is used to carry the signal ground (hence the term isolated ground). 1 and 2 wire sensors do not have a heater.

Titania Sensors

The Titania element in these sensors does not produce a voltage like the Zirconia element. The property of the Titania element which allows for the detection of oxygen in the exhaust gas is its electrical resistance. The electrical resistance of the Titania element changes according to the concentration of oxygen in the exhaust gas. There is a big change in the resistance of the element when the fuel-ratio is 14.7. When a voltage is applied to the element in a voltage divider circuit, the output voltage changes with the resistance thus forming the voltage signal required to be processed by the ECU. As with the Zirconia sensor a low output voltage indicates a lean mixture and a high output voltage indicates a rich mixture. These voltages are used by the ECU for closed loop control.

As Titania sensors do not need air on one side of the element, they can be made smaller and are completely submersible. Due to their different properties Titania and Zirconia sensors should not be interchanged under any circumstance.

Testing Oxygen Sensors

For the ECU to control the AFR and keep it within tight limits, the oxygen sensor must be working properly. Failed or worn out oxygen sensors cause problems such as poor fuel economy, failed emission tests, failure of the catalytic converter and poor driveability. Therefore it is important that you can read the signs of a failed or worn oxygen sensor and have the ability to check their performance.

On Car Test

Before you can test the operation of the sensor, you will need an oscilloscope. You should first check that the basic engine set up is to the manufacturers specification, then thoroughly warm up the engine - remember that the sensor will only function once it has reached its operating temperature.

Two methods of testing an oxygen sensor are using an oscilloscope or a multimeter. An oscilloscope is the best method for testing. This will give you the exact output of the sensor along with its response times. A multimeter can also be used but this will only give an indication if there is an output or no output. The sensor will be switching too quickly for any response times to be measured.

Oscilloscope

Using an appropriate connecting device, connect the sensor output to your oscilloscope; do not disconnect the sensor from the ECU. Run the engine at approximately 2000 rpm. A properly functioning oxygen sensor will show a rapidly fluctuating output voltage between approximately 0.1 and 1.0 volts. The time taken for the voltage to change from 0.1 V to 1.0 V (referred to as the lean to rich response time) should be about 300 milliseconds. A similar time should be measured when the voltage changes from 1.0 V to 0.1 V (rich to lean response time).

Multimeter

For testing with a digital multimeter you will also need to connect the sensor output to the multimeter using an appropriate connecting device. Run the engine at approx. 2000-2500 rpm. The output will be a DC voltage, oscillating between approximately 0.1V and 1.0V. Although the sensor output is technically an oscillating DC voltage some multimeters may require to be set on AC voltage measurement to correctly read the sensor output. Also, the response time of the multimeter must be better than the response time of the sensor. If the multimeter is too slow then a constant output will result even though the sensor is actually switching.

If the sensor output is constant or the response time is too slow the sensor should be changed. It is a good idea to check the oxygen sensor function at every tune up and before submitting cars for emission tests. A slow sensor will affect fuel economy. A new sensor will pay for itself by cutting fuel bills.

Servicing Intervals & Life Expectancy of an Oxygen Sensor

Increasingly stricter, federally mandated emissions regulations demand that you ensure your vehicle's emissions controls are in optimal form and the oxygen sensor plays a vital role in this process. In fact, bad/poisoned oxygen sensors are the leading cause of excess harmful exhaust emissions, contributing to the greenhouse effect.*

Because of the importance, therefore, placed on the service life and role of the oxygen sensor on your vehicle's emissions controls much has been written in recent years by aftermarket manufacturers and suppliers attempting to determine the life expectancy and replacement intervals of oxygen sensors. Real world conditions, however, truly dictate an oxygen sensor's life span. Due to the hostile environment in which sensors operate and the very different circumstances and drivers each vehicle experiences it would be impossible to definitively establish what the service life of a sensor should be.

Oxygen sensors are subjected to a considerable amount of wear & tear, ageing, and extreme temperatures even under normal operating conditions. Harmful contaminants, however, that may be present in your vehicle's exhaust stream can significantly reduce the life span of an oxygen sensor. Factors that contribute to the life span of the oxygen sensor include the location of the sensor on the vehicle's exhaust system. Sensors located directly in the manifold (usually 1 & 2 wire sensors) typically have a shorter life span due to the higher temperatures under which they operate and the increased exposure to harmful exhaust particulates (unspent fuel/oil). Conversely, sensors that have a heater in the thimble element (3 & 4 wire sensors) are more quickly brought up to operating temperatures, and therefore, are thus exposed to less harmful contaminants and operate under lower exhaust temperatures as they can be located further downstream in the exhaust system.

As the world's largest manufacturer and supplier of oxygen sensors, NTK brings to the aftermarket comprehensive knowledge of current and future OEM R & D, technology, and experience. All NTK oxygen sensors are extensively tested during manufacturing to guarantee quality, reliability, and above all, always meet or exceed the original equipment specifications. NTK oxygen sensors are designed for longer service life (from 80,000 up to 150,000km) and have been recognized as a leader in the industry. It is, however, always a good idea to have oxygen sensors checked at each tune-up and/or regular service intervals. With the harsh actual conditions in your vehicle's exhaust coupled with the higher temperatures of today's hotter running engines, NTK generally recommends checking your oxygen sensor at every 60,000km and/or every tune-up for any excess ageing or premature signs of sensor poisoning and contamination. Replace faulty sensors with confidence in a product and name you can trust, NTK Oxygen Sensors.

Sensor Type Description

Single Wire

One wire carries the sensor signal while ground return is achieved through the vehicle body. Also referred to as an EGO Sensor. (Exhaust Gas Oxygen Sensor).

Two Wires

One wire is the sensor signal and the other is the sensor signal ground. Also referred to as an ISO-EGO Sensor (Isolated Exhaust Gas Oxygen Sensor). Both sensor signal wires are directly connected to one of the platinum electrodes on the ceramic element. The sensor output is immune to ground loop voltages and also to large resistances in the vehicle ground return, caused by corroded connections. There are also case grounded sensors that have the ground wire physically attached to the sensor body required for certain applications. It is not recommended to replace isolated ground sensors with case ground types.

Three Wires

One wire carries the signal and two wires are used to supply a voltage to the internal heater. Signal ground return is achieved through the vehicle body. Also referred to as a HEGO Sensor (Heated Exhaust Gas Oxygen Sensor).

Four Wires

One wire is the sensor signal, one is the isolated sensor signal ground and the remaining two are used to supply a voltage to the internal heater. Also referred to as an ISO - HEGO Sensor (Isolated Heated Exhaust Gas Oxygen Sensor).

Five Wires

UEGO (Universal A/F Heated Exhaust Gas Oxygen Sensor) Wide-band heated oxygen that expands upon the 'planar' design of most four wire sensors, actually measuring the Air/Fuel ratio. Instead of switching back & forth like conventional sensors, the wide-band O2 sensor detects a wide range of air to fuel ratio & produces an output signal directly proportional to the A/F ratio. This advanced sensor is free of reference air, has a much higher accuracy and offers precise & tighter control within the stoichometric point for lean-burn applications.

Diagnostic Testing for Oxygen Sensors

Oxygen Sensor Inspection & Analysis

Visual inspection by itself is not usually sufficient to determine if an oxygen sensor is functioning correctly however, the lead wire and connector should be checked for damage. Any damage will interfere with the sensor signal. The sensor body should be checked for dents, which are a sign of mechanical shock that can crack the sensor element. Also, the appearance of the sensor's protection tube can give an indication of possible problems. Below are a few examples of damaged sensors and an indication of what might have been the possible cause. The cause should be rectified and the sensor changed to avoid further problems including damage to the catalytic converter.